The present invention relates to a novel liquid double alkoxide of niobium or tantalum and an alkaline earth metal, a production method thereof, and a production method of a complex metal oxide dielectric using thereof.
Many dielectric thin films made of a complex metal oxide of niobium and/or tantalum and an alkaline earth metal show a high dielectric constant and a ferroelectricity. For example, (SrpBaq)Nb2O6 (wherein, p+q=1) has been investigated and developed as an optical modulator and a infrared detector and SrBi2(TaxNby)2O9 (wherein, x+y=1) has been investigated and developed as a nonvolatile memory, and they are partially used. As a method of producing these thin films using metal organic compounds as the raw materials, there are a sol-gel method, an MOD (Metalorganic deposition) method, and a CVD (Chemical vapor deposition) method. It is known that to make the film composition of an alkaline earth metal and niobium and/or tantalum a theoretical ratio and to crystallize at a lower temperature, it is effective to use a double alkoxide similar to the structure of an oxide crystal in an atomic order as the raw material.
As an example of the sol-gel method and the MOD method used in a solution state, Japanese Patent Laid-Open No. 80181/1999 (Production method of precursor for bismuth layer-structured perovskite ferroelectric thin film) discloses xe2x80x9ca method of producing an Srxe2x80x94Bixe2x80x94Ta or xe2x80x94Nb-based composite alkoxide of Sr:Bi:Ta or Nb 1:2:2 (metal atomic ratio) having structure-controlled of an atomic arrangement, characterized in that an Srxe2x80x94Bi double alkoxide Sr[Bi(OR)4]2 is formed by reacting an Sr alkoxide (Sr(OR)2) prepared from an Sr metal and a Bi alkoxide, (Bi(OR)3) in an alcohol, and then, the product is reacted with a Ta alkoxide Ta(OR)5 or an Nb alkoxide Nb(OR)5xe2x80x9d. In the invention, by analyzing the molecular structure by 1H-NMR, 13C-NMR, 93Nb-NMR, and FT-IR about the product obtained by drying the composite alkoxide solution using a rotary vacuum pump at a temperature of from 50 to 60xc2x0 C., and it is concluded that the composite alkoxide is formed. Also, Sr[Nb(OC2H5OCH3)6]2 is also disclosed in the above-described patent invention. However, these composite alkoxides are not recovered by distillation but are simple residues after distilling away the solvent. Therefore, all the molecules do not always completely form composite products, and also it is hard to say that the composition ratios are not autonomously determined to be the stoichiometric ratios. Also, it is not described in the specification of the invention whether or not the product can be recovered by distillation as the form of the composite alkoxide.
As examples of the CVD method using in a vapor-phase state, Japanese Patent Laid-Open No. 339716/1996(Production method of bismuth layered ferroelectric thin film) discloses double isopropoxides such as Sr[Ta(OCH(CH3)2)6]2, etc., Japanese Patent Laid-Open No. 77592/1997 (Production method of bismuth layered ferroelectric thin film) discloses double ethoxides such as Sr[Ta(OC2H5)6]2, etc., and Japanese Patent Laid-Open No. 102254/1998 (Production method of tungsten bronze-type oxide dielectric thin film) discloses Sr[Nb(OCH(CH3)2)6]2 Sr[Nb(OC2H5)6]2, etc. All these double alkoxides are recovered by distillation or sublimation. However, these compounds are liquids at about 150xc2x0 C. and solids at room temperature as shown in Table 1 below, which become defects in the production and the purification of the compounds and handling property as the raw material for CVD. In addition, it is reported by R. C. Mehrotra (Advances in Inorganic Chemistry and Radiochemistry, Vol. 26, 326(1983)) that these double isopropoxides are all solids and double ethoxides are viscous liquids excluding Ba-based ones, but the compounds synthesized and distilled by the present inventors are solids and the melting points are shown in Table 1 below.
Furthermore, because at a temperature of at least about 180xc2x0 C. having the vapor pressure of at least 0.1 Torr required for CVD, a thermal dissociation of a double alkoxide occurs little by little by the following formula II, the film composition is liable to be apart from the stoichiometric ratio.
Sr[Ta(OC2H5)6]2xe2x86x92[Sr(OC2H5)2]∞+[Ta(OC2H5)5]2xe2x80x83xe2x80x83II
Because [Sr(OC2H5)2]∞ is a polymer and does not have volatility and [Ta(OC2H5)5]2 has a high vapor pressure, the [Ta(OC2H5)5]2 is intermixed with the vapor of Sr[Ta(OC2H5)6]2 and thus Ta/Sr of the vapor phase components becomes larger than 2. In order to remove this defect, the composition and the method disclosed in Japanese Patent Laid-Open No. 298760/1998 (Metal double alkoxide composition for vapor phase growth, supplying method thereof, and production method of complex metal oxide thin film using it) became necessary.
Furthermore, as a novel liquid compound for restraining the thermal dissociation of Sr[Ta(OC2H5)6]2, T. J. Leedham et al., 11th International Symposium on Integrated Ferroelectrics, Abstracts xe2x80x9cNovel precursors for the MOCVD of ferroelectric thin filmsxe2x80x9d, 130C (1998. 3. 7) reported Sr[Ta(OC2H5)5(OC2H4N(CH3)2)]2. However, the detailed identification, production method, and properties thereof have not yet been described. That is, Srxe2x80x94Nb-based, Baxe2x80x94Nb-based, and Baxe2x80x94Ta-based double alkoxides which are liquid compounds at room temperature and are hard to be thermally dissociated are not known.
The present invention provides a novel double alkoxide of niobium or tantalum and an alkaline earth metal, which is hard to be thermally dissociated, is a liquid at room temperature, and has a property capable of being distilled. Furthermore, the invention provides a production method of the compound and also provides a production method of a complex metal oxide dielectric using the above-described compound.
The present inventors have found that a novel double alkoxide represented by the following formula I is hard to be thermally dissociated, is a liquid at room temperature, and has the property capable of being distilled;
M[N(OC2H5)5(OC2H4OR)]2xe2x80x83xe2x80x83I
wherein, M represents Sr or Ba, N represents Nb or Ta, and R represents an alkyl group having from 1 to 4 carbon atoms. In the formula I, the example of R is methyl, ethyl, propyl, isopropyl, or butyl but is preferably methyl.
The compound of this invention shown by the formula I can be easily produced by reacting N(OC2H5)5 and M(OC2H4OR)2. Also, the compound shown by the formula I wherein R is methyl can be easily produced by reacting N(OC2H5)5 and M(OC2H4OCH3)2.
The compound of this invention shown by the formula I can be easily produced by reacting N(OC2H5)5 and M(OC2H4OR)2 and then recovering by distillation and the compound obtained has a correct stoichiometric ratio and a property of high purity.
Also, the compound of this invention shown by the formula I wherein R is methyl can be easily produced by reacting N(OC2H5)5 and M(OC2H4OCH3)2 and recovering by distillation, and the compound obtained has a correct stoichiometric ratio and has a property of high purity.
Furthermore, in this invention, a complex metal oxide dielectric MN2O6 thin film can be easily produced using the compound of this invention shown by the formula I.
Also, in this invention, a complex metal oxide dielectric MN2O6 thin film can be easily produced using the compound of this invention shown by the formula I wherein R is methyl.
Moreover, in this invention, a complex metal oxide dielectric MN2O6 thin film can be easily produced by CVD using the compound of this invention shown by the formula I.
Also, in this invention, a complex metal oxide dielectric MN2O6 thin film can be easily produced by CVD using the compound of this invention shown by the formula I wherein R is methyl.
Furthermore, in this invention, a dielectric thin film shown by (SrpBaq)Nb2O6 (wherein p+q=1) can be easily produced using the compounds shown by Sr[Nb(OC2H5)5(OC2H4OR)]2 and Ba[Nb(OC2H5)5(OC2H4OR)]2 (in both formulae, R represents an alkyl group having from 1 to 4 carbon atoms).
Also, in this invention, a dielectric thin film shown by (SrpBaq)Nb2O6 (wherein p+q=1) can be easily produced using Sr[Nb(OC2H5)5(OC2H4OCH3)]2 and Ba[Nb(OC2H5)5(OC2H4OCH3)]2.
Still further, in this invention, a dielectric thin film shown by (SrpBaq)Nb2O6 (wherein p+q=1) can be easily produced by CVD using the compounds shown by Sr[Nb(OC2H5)5(OC2H4OR)]2 and Ba[Nb(OC2H5)5(OC2H4OR)]2 (in both formulae, R represents an alkyl group having from 1 to 4 carbon atoms).
Furthermore, in this invention, a dielectric thin film shown by (SrpBaq)Nb2O6 (wherein p+q=1) can be easily produced by CVD using Sr[Nb(OC2H5)5(OC2H4OCH3)]2 and Ba[Nb(OC2H5)5(OC2H4OCH3)]2.
Also, in this invention, a bismuth layered ferroelectric thin film shown by SrBi2(TaxNby)2O9 (wherein x+y=1) can be easily produced using the compounds shown by Sr[Ta(OC2H5)5(OC2H4OR)]2 and Sr[Nb(OC2H5)5(OC2H4OR)]2 (in both formulae, R represents an alkyl group having from 1 to 4 carbon atoms).
Furthermore, in this invention, a bismuth layered ferroelectric thin film shown by SrBi2(TaxNby)2O9 (wherein x+y=1) can be easily produced using Sr[Ta(OC2H5)5(OC2H4OCH3)]2 and Sr[Nb(OC2H5)5(OC2H4OCH3)]2.
Also, in this invention, a bismuth layered ferroelectric thin film shown by SrBi2(TaxNby)2O9 (wherein x+y=1) can be easily produced by CVD using the compounds shown by Sr[Ta(OC2H5)5(OC2H4OR)]2 and Sr[Nb(OC2H5)5(OC2H4OR)]2 (in both formulae, R represents an alkyl group having from 1 to 4 carbon atoms).
Furthermore, in this invention, a bismuth layered ferroelectric thin film shown by SrBi2(TaxNby)2O9 (wherein x+y=1) can be easily produced by CVD using Sr[Ta(OC2H5)5(OC2H4OCH3)]2 and Sr[Nb(OC2H5)5(OC2H4OCH3)]2.